Understanding the in vivo regulation of metabolism is central in determining nutritional requirements in health and disease. The application of tracer methodology and mathematical modeling to this topic has already yielded much important information. Mathematical modeling is essential in studies of metabolic regulation in human subjects, since it is virtually impossible to directly measure any metabolic processes. Application of sophisticated modeling techniques in the field of metabolism has been largely limited by the lack of expertise of physiologists in complicated computational techniques. In contrast, sophisticated mathematical modeling lies at the heart of many aspects of engineering. Many new technologies have resulted from collaborations between physiologists and engineers, but such collaborations in the field of metabolism have been limited.